1. Field of the Invention
The present invention relates to a method for forming a film having a thickness of 20 μm or more on a surface of a semiconductor wafer or a glass substrate.
2. Description of the Prior Art
Recently, trials of forming a thick resist film on a surface of a substrate have been performed. For example, in order to form a protruding electrode, which is referred to as a bump, having a height of around 20 μm on a surface of an IC pattern by applying integrated circuit forming technology, a resist film having a thickness of around 20 μm is formed on a surface of a substrate, exposure is conducted to this resist film through a mask, and development is conducted so as to provide an opening in the area to be formed into a bump. Next, this opening is filled with metal by plating or CVD, and thereafter an ashing process is conducted to the resist film so as to finally form a bump.
In addition, wire bonding, which has conventionally been used for mounting an IC chip on a substrate, requires labor and time because it is necessary to connect metal wires one by one in wire bonding. Thus, instead of wire bonding, there is another way, in which a plurality of metal posts are provided on a chip, and the chip is mounted on a substrate via the posts. The metal posts have a height of around 100 μm, and the metal posts are formed by the same method as mentioned above.
As a method for forming a resist film in which a resist solution is applied onto a substrate to be treated, and thereafter a solvent within the resist solution is removed, Document 1 has disclosed that a substrate to be treated on which a resist solution has been applied is mounted directly on a hot plate so as to be heated.
[Document 1] Japanese Patent Application Publication No.2002-324745, paragraph 0017
FIG. 5 (a) shows a state where a substrate on which a resist solution has been applied is mounted on a hot plate. In this instance, the thickness of the resist solution is initially uniform. Then, the substrate and the resist solution are heated, which causes convection to the resist solution due to the temperature difference between the lower side and the upper side of the resist solution. As shown in FIG. 5 (b), the convection moves the resist solution still having flowability toward the outside so as to form a bead portion (projection) at the edge.
As for the shape of the bead portion, it is slightly projected in an area about 5 mm from the outer periphery, its thickness is then gradually decreased toward the outside, and finally the thickness is significantly increased in the most outside area. It is assumed that the reason why the bead portion has two stages is the influence of the convection. However, the precise reason is unknown. In any event, when such a bead portion has been formed, it becomes necessary to remove the inside projection and the significantly increased thickness at the most outside area thereof by using a rinse liquid. Consequently, an effective area for forming a circuit is undesirably decreased.
In a case of the relatively small thickness of a common resist film, even if only one side is heated, the temperature difference between the lower side and the upper side of the resist solution is small, and no convection so as to form a bead portion occurs. However, in the case of the resist film for forming a bump or metal posts, since the thickness is 20 μm or more, convection easily occurs.